HOW TO: Implement Group Validation in ASP.NET 1.X
Solution
Simulating ValidationGroup (of ASP.NET 2.0) feature in ASP.NET 1.X
by Sreedhar Koganti - The concept here is to selectively enable/disable validation controls on a page using client-side scripting (Javascript).
ASP.NET validation in groups
by Christian Tratz (CodeProject.com) - This solution requires disabling client-side validation for uplevel browsers.
Related Resources
MSDN: User Input Validation in ASP.NET
MSDN: ASP.NET Validation in Depth